Conventional X-ray imaging systems comprise an X-ray source and an array of detectors. Some or all of the X-rays produced in the X-ray source fall on the detector array after passing through an object being scanned such as baggage or cargo. Commonly used X-ray cargo inspection systems use a pulsed linear-accelerator-based X-ray source, which emits a Bremsstrahlung spectrum of X-rays. The X-rays that penetrate the cargo are detected by detectors that usually consist of scintillator crystals (e.g. CdWO4) with photodiode read-outs. During typical pulse durations of a few microseconds, tens to hundreds of thousands of X-rays arrive at each detector, except for those that are absorbed or scattered by the cargo.
The spectrum of X-rays arriving at the detectors after passing through the cargo material being scanned possesses different characteristics with respect to different materials. In other words, the resultant spectrum of X-rays is material specific. Lower-energy X-rays are absorbed more readily in the cargo than higher-energy ones mainly due to the photoelectric effect. High-energy X-rays can also be lost, mainly due to Compton scattering and electron-positron pair-production. The cross sections for these processes, a measure of their probability, depend on the energy of the X-rays and on the atomic number (Z) of the cargo material. In particular, the cross section for pair-production increases with the Z of the material. Therefore, after X-rays have traversed high-Z materials, the energy of the remaining X-rays is, on average, lower than after they have traversed equivalent quantities of lower-Z materials: relatively, more X-rays at the high end of the spectrum are lost.
Hence, it is possible to determine the type of material traversed by the X-rays if the spectrum of the X-rays arriving in the detectors is measured with sufficient precision. Spectroscopic techniques may, in principle, be used for measurement of the energy spectrum of the X-rays. Minimum count rates for standard systems with pulsed X-ray sources, for example count rates at “air value” (where there is nothing in the X-ray beam), are typically on the order of several to tens of millions of X-rays per second, for a low dose configuration but can be as high as several to tens of billions of X-rays per second in high energy, high-dose systems. Therefore, the X-ray count rates are usually too high to be measured using standard spectroscopy methods, where the energy of each arriving X-ray is measured individually.
Further, the scintillation detectors usually employed in conventional X-ray scanning systems are much too slow. For example, CdWO4 has a ˜15 μsec decay time, which is longer than the typical X-ray pulse itself. Hence, the detector would be unable to detect separate signals for each of the many individual X-rays arriving in the same pulse. Therefore, X-ray detectors used in most radiography applications function by measuring the total X-ray energy transmitted, as opposed to the energies of individual X-rays detected. This is sometimes called “operating in integration mode”.
Hence, there is need for a method of detecting the presence of materials based on their atomic numbers by spectroscopic analysis in a manner that takes advantage of the information provided by discrete X-ray transmissions, yet can be implemented by conventional detector systems.